5.5 Astrophysics and cosmology Flashcards
Define planet (3 main points).
An object that:
1) Is in orbit around the sun.
2) Has a mass sufficient for its own gravity to force it to it a spherical shape
3) Has cleared its orbit of other objects.
Define planetary satellites.
Naturally-formed bodies that orbit planets.
Define comet.
A body formed of ice and rock which travels in an elliptical orbit around the sun.
Define solar system.
The system encompassing a star and the gravitationally bound objects which orbit it.
(Our solar system has one star - the sun, eight planets, five officially named dwarf planets, hundreds of moons, thousands of comets, and more than a million asteroids.)
Define galaxy.
A cluster of billions of stars held together by gravity. Earth and the Solar System are located in a galaxy called the Milky Way.
Define the universe.
All existing matter and space considered as a whole.
Define nebulae.
Gigantic clouds of interstellar (between stars) dust and gas; the birthplace of all stars.
How does a star form? Outline the steps of gravitational collapse, fusion of hydrogen into helium, radiation pressure, gas pressure and stabilisation.
Gravitational Collapse:
Nebulae are pulled together by gravity. As the material collapses, it heats up and forms a protostar.
Fusion of Hydrogen into Helium:
When the core temperature of the protostar becomes high enough, nuclear fusion begins.
Hydrogen nuclei fuse to form helium, releasing energy.
Radiation Pressure:
Energy from fusion creates radiation that pushes outward, counteracting the gravitational collapse.
Gas Pressure:
The heated gas inside the star exerts pressure that also supports against further collapse.
Stabilization:
A balance between gravitational collapse, radiation pressure, and gas pressure results in a stable main sequence star.
Describe the evolution of a low or high mass star into a red giant.
- Most of the hydrogen nuclei in the core of the star have been fused into helium and so nuclear fusion slows and the energy released by fusion decreases.
- The inward gravitational force becomes greater than the outward force from the gas pressure and radiation pressure.
- The core collapses, leading to an increase in temperature as it compresses under the weight of the star.
- Fusion in the core stops.
- The outer layers of the star expand and then cool forming a red giant.
Describe the evolution of a low mass star (such as the sun) from being a red giant into a white dwarf.
- In low mass stars, the core isn’t hot enough for any further fusion and so it continues to contract under its own weight. Once the core has shrunk to about Earth size, electrons exert enough pressure (electron degeneracy pressure) to stop it from collapsing any more.
- This only works for stars with a core mass under roughly 1.4 times the mass of the sun, otherwise the electron degeneracy pressure win’t be enough to counterract the gravitational force and the star will collapse inwards.
Define planetary nebula.
A region of cosmic gas and dust formed from the cast-off outer layers of a dying star.
Define electron degeneracy pressure
When electrons exert pressure to support white dwarfs against gravitational collapse
Define the Chandrasekhar limit
The maximum mass that a star can have before it collapses under its own gravity
Describe the characteristics of a white dwarf.
- Extremely dense and hot at first, but gradually cools and dims over time.
- No nuclear fusion; supported by electron degeneracy pressure.
Describe the evolution of a massive red super giant into a neutron star or black hole.
In the red supergiant, fusion continues in shells around the core which produces heavier elements like iron.
Iron cannot undergo fusion to release energy, so the core becomes unstable. The core’s gravity overwhelms the outward pressure from fusion, causing a rapid collapse.
The collapse triggers a violent supernova, where the outer layers are expelled into space. The core temperature increases, leading to nuclear reactions that may form elements heavier than iron.
Neutron star
After the supernova explosion, the collapsed neutron core can remain intact, known as a neutron star.
Black Hole
If the remaining core mass exceeds approximately 3 solar masses, no known force can stop the collapse. The core continues collapsing to a singularity, forming a black hole.
Describe the characteristics of a neutron star
A highly dense, rapidly rotating remnant, often emitting X-rays or radio waves.
Describe the characteristics of a black hole.
Gravity is so strong that nothing, not even light, can escape as the escape velocity of the core is greater than the speed of light This is because matter is packed into a very small space.
